Antibodies against bovine type I and type II cAMP-dependent protein kinase regulatory subunits RI and RII, respectively, were raised in rabbits and were purified by utilizing a new cross-linked affinity chromatography procedure. These antibodies specifically cross reacted with the human breast cancer cells (MCF-7 and MDAMB-231) as determined by radioimmunoassay and immunoprecipitation. The immunocytochemical studies utilizing these antibodies revealed that RII, but not RI, is specifically compartmentalized in the nucleoli and mitototic spindles of these cancer cells, suggesting that RI and RII are involved in the different nuclear directed events in cancer cells. The photo-affinity labelling of the R proteins of MCF-7 cells revealed the prsence of 50 K isoprotein of type II cAMP-dependent kinaSE. The physiological function of the 50 protein in relation ;to its subcellular compartmentalization during different phases of growth is currently under investigation.